A separation membrane is used in a wide variety of fields such as desalination of seawater and brackish water, production of pure water and ultrapure water for medical/industrial use, effluent treatment, and the food industry. In separation treatment through the separation membrane, a separation membrane element using the separation membrane is widely used. In the separation membrane element, the separation membrane, a liquid-to-be-treated channel member, and a permeated liquid channel member are spirally wound around a permeated liquid collection tube, and anti-telescoping plates are disposed on both ends of the wound membrane body (for example, Patent Document 1).
The conventional technique is described with reference to FIGS. 1 to 4.
FIG. 1 is a schematic view illustrating a membrane separation system using a separation membrane element in the conventional technique. FIG. 2 is a partially cutout perspective view of the separation membrane element. A plurality of separation membrane elements 1 are individually loaded in series in a pressure vessel 2. Here, adjacent separation membrane elements 1 are interconnected with an interconnection member 4 which is freely inserted into and removed from a permeated liquid collection tube 12 which will be described in FIG. 2. Liquid to be treated (untreated liquid) is fed to the pressure vessel 2 in a state of being pressurized by a pump 3 and is separated into concentrated liquid and permeated liquid, through the separation membrane element 1. Permeated liquid adaptors 5 of the pressure vessel 2 both on the liquid-to-be-treated feed side and on the concentrated liquid discharge side are connected to the permeated liquid collection tube 12 of the separation membrane elements 1, and it is possible to discharge the permeated liquid obtained through the separation membrane elements 1 to the outside from the pressure vessel 2.
Each of the separation membrane elements 1 includes a brine seal 14 as a member which seals a gap between the pressure vessel 2 and the separation membrane element. Further, a thrust load holding member 7 is disposed most downstream in the pressure vessel 2 so as to hold a thrust load generated due to pressure drop occurring when the liquid to be treated flows through the separation membrane elements 1. Therefore, in general, a mechanism, in which the separation membrane element 1 positioned most downstream is brought into contact with the thrust load holding member without a gap, is formed. In the separation membrane element 1, a separation membrane 9 in which a permeated liquid channel member 10 is involved is spirally wound around the permeated liquid collection tube 12 through a liquid-to-be-treated channel member 11, and anti-telescoping plates 13 are disposed on both ends of the wound body.
In addition, the pressure vessel 2 is often manufactured so as to have an extra space of about 5 mm to 30 mm added to a total length calculated from the number of loaded separation membrane elements 1 such that a plurality of separation membrane elements 1 can be loaded, even when there is a total length tolerance of the separation membrane elements 1. However, when the separation membrane elements 1 are operated in a state where such a gap is remained as it is, the separation membrane elements 1 move or vibrate within a range of the extra space due to a change in pressure or flow occurring when the operation is started or finished, which results in damage to the permeated liquid adaptor 5 and damage to a seal member 18 mounted on the permeated liquid adaptor 5. In order to prevent the damage, a plurality of movement preventive members 8 having a thickness of 1 mm to 5 mm are mounted on the permeated liquid adaptor 5 so as to fill the extra space for preventing the separation membrane elements 1 from moving.
When a membrane separation treatment, in which a liquid to be treated passes through the separation membrane elements 1 loaded in the pressure vessel 2, is continually performed, contaminants in the liquid to be treated are attached and become accumulated on a membrane surface in the separation membrane element 1. Therefore, a function of the separation membrane element 1 deteriorates, and performance deterioration such as a water permeability decrease or water quality degradation of the permeated liquid occurs. In addition, a channel used when the liquid to be treated flows through the separation membrane element 1 is narrowed due to the contaminants, thereby increasing pressure drop, in some cases. In particular, the contaminants are often attached and become accumulated on the membrane surface on the liquid-to-be-treated side of the separation membrane element 1 positioned most upstream in the pressure vessel 2.
Conventionally, when the contaminants are attached and become accumulated on the membrane surface in the separation membrane element 1, thereby resulting in performance deterioration of the separation membrane element 1, in general, in order to remove the contaminants attached on the membrane surface, a method (forward flushing) in which a flushing liquid such as an acidic solution or an alkaline solution is caused to flow into the separation membrane element 1 from the upstream side to the downstream side of the pressure vessel 2, namely in the same direction as the direction of the membrane separation treatment, and the membrane surface is cleaned, has been used for recovering the performance of the separation membrane element 1.
However, in the cleaning method described above, a problem arises in that not only it is not possible to discharge, to the outside of the system, the contaminants attached to the separation membrane element 1 on the upstream side because the contaminants are caught along with the flushing liquid inside the separation membrane element 1 on the downstream side which is not contaminated with the contaminants, but also the separation membrane element 1 on the downstream side becomes contaminated. Hence, cases of using a cleaning method (backward flushing) in which the flushing liquid flows from the concentrate discharge side to the liquid-to-be-treated feed side of the pressure vessel 2, namely, in the backward direction opposite to the direction of the normal membrane separation treatment, or a cleaning method in which the forward flushing and the backward flushing are combined, are increased.
In a case of performing such backward flushing, among the separation membrane elements 1 in the pressure vessel 2, a thrust load generated due to the pressure drop occurring during the backward flushing is applied to the separation membrane element 1 on the liquid-to-be-treated feed side; however, since the thrust load holding member 7 for holding the thrust load is not disposed on the liquid-to-be-treated feed side of the pressure vessel 2, the thrust load is applied to the permeated liquid adaptor 5 and the permeated liquid collection tube 12 on the liquid-to-be-treated feed side, thereby resulting in damage to these members. Therefore, in order to apply the backward flushing, the same thrust load holding member 7 as that on the concentrated liquid discharge side also needs to be disposed on the liquid-to-be-treated feed side in the pressure vessel 2.